Advances in understanding of the requirements for tumor antigen recognition and immune effector function indicate that a potential strategy to enhance an anti-tumor immune response is to provide co-stimulation through an auxiliary molecule. The current model for T-cell activation postulates that naive T-cells require two signals for full activation: (i) a signal provided through the binding of processed antigens presented to the T-cell receptor by major histocompatibility complex (MHC) class I molecules; and (ii) an additional signal provided by the interaction of co-stimulatory molecules on the surface of T-cells and their ligands on antigen presenting cells.
CD137 (4-1BB) is a member of the TNF receptor superfamily and is an activation-induced T-cell costimulatory molecule. The receptor was initially described in mice (B. Kwon et al., P.N.A.S. USA, 86:1963-7 (1989)), and later identified in humans (M. Alderson et al., Eur. J. Immunol., 24: 2219-27 (1994); Z. Zhou et al., Immunol. Lett., 45:67 (1995)). The interaction of CD137 and the CD137 ligand (4-1BBL) activates an important costimulatory pathway. Signaling via CD137 upregulates survival genes, enhances cell division, induces cytokine production, and prevents activation-induced cell death in T cells. The importance of the CD137 pathway has been underscored in a number of diseases, including cancer (see, e.g., U.S. Pat. No. 7,288,638).
Expression of CD137 has been shown to be predominantly on cells of lymphoid lineage such as activated T-cells, activated Natural Killer (NK) cells, NKT-cells, CD4CD25 regulatory T-cells, and also on activated thymocytes, and intraepithelial lymphocytes. In addition, CD137 has also been shown to be expressed on cells of myeloid origin like dendritic cells, monocytes, neutrophils, and eosinophils. Even though CD137 expression is mainly restricted to immune/inflammatory cells, there have been reports describing its expression on endothelial cells associated with a small number of tissues from inflammatory sites and tumors.
The physiological events observed following CD137 stimulation on T-cells are mediated by NF-κB and PI3K/ERK1/2 signals with separate physiological functions. NF-κB signals trigger expression of Bcl-XL, an anti-apopotic molecule, thus resulting in increased survival, whereas PI3K and ERK1/2 signals are specifically responsible for CD137-mediated cell cycle progression (H. Lee et al., J. Immunol., 169(9):4882-8 (2002)). The effect of CD137 activation on the inhibition of activation-induced cell death was shown in vitro by Hurtado et al. (J. Hurtado et al., J. Immunol., 158(6):2600-9 (1997)), and in an in vivo system in which anti-CD137 monoclonal antibodies (mabs) were shown to produce long-term survival of superantigen-activated CD8+ T-cells by preventing clonal deletion (C. Takahashi et al., J. Immunol., 162:5037 (1999)). Later, two reports demonstrated, under different experimental conditions, that the CD137 signal regulated both clonal expansion and survival of CD8+ T-cells (D. Cooper et al., Eur. J. Immunol., 32(2):521-9 (2002); M. Maus et al., Nat. Biotechnol., 20:143 (2002)).
Altogether, CD137 stimulation results in enhanced expansion, survival, and effector functions of newly primed CD8+ T-cells, acting, in part, directly on these cells. Both CD4+ and CD8+ T-cells have been shown to respond to CD137 stimulation, however, it appears that enhancement of T-cell function is greater in CD8+ cells ((W. Shuford et al., J. Exp. Med., 186(1):47-55 (1997); I. Gramaglia et al., Eur. J. Immunol., 30(2):392-402 (2000); C. Takahashi et al., J. Immunol., 162:5037 (1999)). Based on the critical role of CD137 stimulation in CD8+ T-cell function and survival, agonism of the CD137/CD137L system provides a plausible approach for the treatment of tumors and viral pathogens.
Alternatively, while it has been shown that agonistic antibodies to CD137 and the ligand to CD137 enhance lymphocyte activation, the CD137 protein has the opposite effect. It inhibits proliferation of activated T lymphocytes and induces programmed cell death. These T cell-inhibitory activities of CD137 require immobilisation of the protein, arguing for transmission of a signal through the ligand/coreceptor (Schwarz et al., Blood 87, 2839-2845 (1996); Michel et al., Immunology 98, 42-46 (1999)).
The known human CD137 ligand is expressed constitutively by monocytes and its expression is inducible in T lymphocytes (Alderson et al., Eur. J. Immunol. 24, 2219-2227 (1994)). Monocytes are activated by immobilised CD137 protein and their survival is profoundly prolonged by CD137. (Langstein et al., J. Immunol. 160, 2488-2494 (1998); Langstein et al., J. Leuk. Biol. 65, 829-833 (1999)). CD137 also induces proliferation in peripheral monocytes (Langstein et al., 1999b). Macrophage colony-stimulating factor (M-CSF) is essential for the proliferative and survival-enhancing activities of CD137 (Langstein et al., J. Leuk. Biol. 65, 829-833 (1999); Langstein et al., Blood 94, 3161-3168 (1999)).
Signalling through CD137 ligand has also been demonstrated in B cells where it enhances proliferation and immunoglobulin synthesis. This occurs at interactions of B cells with CD137-expressing T cells or follicular dendritic cells (Pauly et al., J. Leuk. Biol. 72, 35-42 (2002)). It was postulated that similarly to the CD40 receptor/ligand system, which mediates T cell help to B cells after first antigen encounter, the CD137 receptor/ligand system may mediate co-stimulation of B cells by FDC during affinity maturation (Pauly et al., J. Leuk. Biol. 72, 35-42 (2002)).
Furthermore, soluble forms of CD137 are generated by differential splicing and are selectively expressed by activated T cells (Michel et al., Eur. J. Immunol. 28, 290-295 (1998)). Soluble CD137 is antagonistic to membrane-bound or immobilised CD137, and levels of soluble CD137 correlate with activation induced cell death in T cells (DeBenedette et al., J. Exp. Med. 181, 985-992 (1995); Hurtado et al., J. Immunol. 155, 3360-3367 (1995); Michel et al., Cytokine 12, 742-746 (2000)).
Thus, considering the complicated picture for CD137 involvement in divergent mechanisms of action and different cell types, there exists a need for reagents to accurately detect CD137 in tumor tissues and other samples and for new therapeutics that modulate the interaction between CD137 and the 4-1BBL ligand or that modulate the interaction between CD137 and other cellular targets.